Hydrogenation of Ethyl Acetate to Ethanol over Ni-Based Catalysts Obtained from Ni/Al ...

14 Pages·2010·0.53 MB·English

by  Beixiao Zhang

Molecules 2010, 15, 5139-5152; doi:10.3390/molecules15085139

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molecules
ISSN 1420-3049
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Article
Hydrogenation of Ethyl Acetate to Ethanol over Ni-Based
Catalysts Obtained from Ni/Al Hydrotalcite-Like Compounds
Beixiao Zhang, Lu Lin *, Junping Zhuang, Ying Liu *, Lincai Peng and Longfei Jiang
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,
Guangzhou 510640, Guangdong, China
* Author to whom correspondence should be addressed; E-Mails: [email protected] (L.L.);
[email protected] (Y.L.).
Received: 18 June 2010; in revised form: 17 July 2010 / Accepted: 23 July 2010 /
Published: 29 July 2010

Abstract: A series of Ni-based catalysts were prepared using hydrogen reduction of Ni/Al
hydrotalcite-like compounds (Ni/Al HTlcs) synthesized by coprecipitation. The physico-
chemical properties of Ni/Al hydrotalcite-like compounds and the corresponding Ni-based
catalysts were characterized using inductively coupled plasma (ICP), BET surface areas, X-
ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning
electron microscopy (SEM) techniques. The results indicated that Ni/Al HTlcs with layered
structures could be successfully prepared by the coprecipitation method, and the
characteristic HTlcs reflections were also observed in the XRD analysis. The NiO and Ni0
phases were identified in all Ni-based catalysts, which displayed randomly interconnected
pores and no layer structures. In addition, the studies also found the Ni/Al HTlcs and Ni-
based catalysts had high specific surface areas, low pore volumes and low pore diameters.
The catalytic hydrogenation of ethyl acetate to ethanol with Ni-based catalysts was also
investigated. Among the studied catalysts, RE1NASH-110-3 showed the highest selectivity
and yield of ethyl acetate to ethanol, which were 68.2% and 61.7%, respectively. At the
same time, a major by-product, butyl acetate, was formed due to an ester-exchange
reaction. A proposed hydrogenation pathway for ethyl acetate over Ni-based catalysts was
suggested.
Keywords: hydrogenation; ethyl acetate; ethanol; Ni/Al hydrotalcite-like compounds
Molecules 2010, 15 5140

1. Introduction
Exploitation of diverse methods to produce fuel ethanol from lignocellulose is meaningful for the
research and development activities in the alternative energy field. Ethyl acetate can be produced from
lignocellulosic biomass via transformation of glucose to levulinic acid. There are two steps in the
transformation of levulinic acid into ethyl acetate [1,2], one is the decarboxylation of levulinic acid to
butanone [3], the other is Baeyer-Villiger oxidation of butanone to ethyl acetate [4]. Ethyl acetate can
then be further reduced to ethanol (Scheme 1). As one kind of potential vehicle fuel, fuel ethanol is
useful to reduce the consumption of gasoline all over the world, and will provide much more
sustainable options for the production of biomass fuels.
Scheme 1. The pathway of conversion of lignocellulose to ethanol via levulinic acid.
CH2OH OH CH2OH O
HOHO O OHO O OHO O OH hydrolysis OH decarboxylation O
OH CHOH n-2 OH O
2
Cellulose
Levulinic acid Butanone
O
Baeyer-Villiger oxidation hydrogenation
O OH
Ethyl acetate Ethanol

Reduction of esters to the corresponding alcohols is one of the fundamental reactions in organic
chemistry, and is employed in a large number of chemical processes. In general, there are two main
types of processes to achieve such a reduction. One is through traditional hydride processes, in which
silyl or metal hydride salts, such as LiAlH4 and NaBH4 are used, the other is through hydrogenation
processes, where molecular hydrogen is employed. From a practical point of view, hydrogenation
processes can be run with small amounts of catalyst and in the presence of small quantities or even in
the absence of solvent. Furthermore, hydrogenation processes do not require highly reactive and
expensive hydrides, engender less aqueous waste, and are environmentally friendly and economically
attractive [5].
Hydrogenation of esters to the corresponding alcohols is usually carried out according to the
stoichiometry shown in Equation 1:
R COOR + 2H = R CH OH+ R OH (1)
1 2 2 1 2 2
Various catalysts have been successfully used in these hydrogenation reactions, including both
homogeneous and heterogeneous catalysts. Homogeneous catalytic hydrogenation of esters has been
rarely explored and is mostly limited to activated esters [6-8]. Roger and coworkers reported that the
anionic ruthenium hydride complex K [(PPh ) (PPh )Ru H ]·(C H O ) could catalyze the
2 3 3 2 2 4 6 14 3 2
hydrogenation of activated esters, while ethyl acetate was hydrogenated to ethanol with only 8% yield
and aromatic esters could not be hydrogenated [7]. By contrast, heterogeneous catalysts are widely
used in hydrogenation of esters, for example, copper-based catalysts exhibit high activity and
selectivity for hydrogenation of esters to alcohols [9], but the use of the catalysts requires severe
Molecules 2010, 15 5141

reaction conditions at 250–350 °C and approximately 10-20 MPa. Some catalysts based on transition
metals such as Sn, Ru, Pd, Pt, Re and Rh are also reported to be usable in the reduction of esters [10].
Abdellah et al. claimed that the hydrogenation of ethyl acetate to ethanol could be carried out with
various Rh/Sn/SiO catalysts, the highest yield was 68% [11]. Okumura et al. described that Rh/one-
2
atomic-layer GeO /SiO catalysts had exhibited high activity and selectivity for ethyl acetate
2 2
hydrogenation [12].
Hydrotalcite belongs to the large class of anionic clays with layered brucite structures and a general
formula of Mg Al (OH) CO ·4H O. Hydrotalcite-like compounds (HTlcs), also known as layered
6 2 16 3 2
double hydroxides, consist of brucite-like layers with positive charge and anionic compounds in the
interlayer, their general formula is [M(II) M(III) (OH) ]x+(An- )·mH O, where M(II) may be Mg, Zn,
1-x x 2 x/n 2
Co, Cu, Fe, Ni and Mn, M(III) may be Al, Cr, Fe and V, and A is usually CO 2- anions [13]. A wide
3
variety of HTlcs have been synthesized by modification of the lamellar and interlamellar regions.
Due to its large surface area, basic properties, high metal dispersion and stability against sintering,
hydrotalcite-like compounds and their derived forms have recently been attracting considerable
attention for their potential applications in oxidation [14], hydrogenation [15-17], steam reforming
[18], dehydrogenation [19], condensation [20], alkylation [21], Meerwein–Ponndorf–Verley reactions
[22] and so on. Chen et al. investigated the hydrogenation of methyl benzoate to benzaldehyde on
manganese oxide catalysts prepared from Mg/Mn/Al hydrotalcite-like compounds [15], in which the
conversion of methyl benzoate and selectivity to benzaldehyde with a dosage of 3% K/Mg Mn Al
0.2 1.8 1
catalyst at 390 °C were 90.0% and 88.0%, respectively. Boyapati et al. reported that the hydrogenation
of aromatic and heterocyclic aldehydes at atmospheric pressure by calcinated Ni/Al hydrotalcite
generated high yields of the reduced products, and the catalyst could be reused for several cycles with
consistent catalytic activity and selectivity [16].
It is well known that nickel is a cheaper metal compared with the noble metals such as palladium,
rhodium and ruthenium, and it is widely used for hydrogenation of organic compounds. In the present
study, Ni/Al HTlcs were synthesized by a coprecipitation method. The physicochemical properties of
Ni/Al hydrotalcite-like compounds and the corresponding Ni-based catalysts were characterized using
various techniques. The Ni-based catalysts were used for the hydrogenation of ethyl acetate to ethanol.
The proposed hydrogenation pathway for ethyl acetate over Ni-based catalysts was also studied.
2. Results and Discussion
2.1. Sample characterization
The Ni/Al molar ratios of the prepared hydrotalcite-like compounds are shown in Table 1. The
results showed that the actual Ni/Al molar ratio was almost identical to the experimental design value,
which indicated that the precipitation process was efficient, the Ni/Al hydrotalcite-like compounds
with different Ni/Al molar ratio could be successfully prepared by coprecipitation methods.
The specific surface areas (S ), pore volume (Vp) and average pore diameters (dp) of various
BET
samples were studied, as shown in Table 2. The results indicated that the specific surface area and pore
volume of Ni/Al HTlcs increased as the Ni/Al molar ratio increased, which could also be seen in the
resultant reduced products, whereas average pore diameters decreased with the increase of Ni/Al molar
ratio whether the Ni/Al HTlcs were prepared by reflux at 80 °C or maintained under static conditions.
Molecules 2010, 15 5142

When the Ni/Al molar ratio varied from 1:1 to 5:1, the specific surface areas of Ni-based catalysts with
reflux treatment increased from 100.6 to 139.1 m2/g, and ranged from 105.7 to 149.6 m2/g,
respectively, under static conditions. The Vp values drifted from 0.24 to 0.40 mL/g and increased with
the increase of specific surface areas. However, the average pore diameters decreased conversely in the
range from 18.16 to 10.25 nm. The nucleation and growth of HTlcs was a complicated process, and the
crystal formation was attributed to the influence of several factors. The ionic radius of nickel is larger
than that of aluminum, so the incorporation of more nickel could lead to lattice expansion [23], which
might be one of the reasons for the observed changes of S , Vp and dp as the Ni/Al molar ratio
BET
increased. In addition, crystallization conditions and reduction temperatures in the preparation could
also influence the pore structure.
Table 1. The molar ratio of Ni/Al hydrotalcite-like compounds.
Catalyst Ni/Al ratio design Actual Ni/Al ratio
1NAR-80-20 1 1.17
3NAR-80-20 3 3.21
5NAR-80-20 5 5.32
1NASH-110-3 1 1.22
3NASH-110-3 3 3.18
5NASH-110-3 5 5.27
Table 2. Specific surface area and pore structure analysis of the Ni/Al HTlcs and Ni-based
catalysts.
Catalyst S (m2/g)a V (mL/g)b dp(Å)c
BET p
1NASH-110-3 124.6 0.32 16.07
5NASH-110-3 169.3 0.43 9.35
RE1NAR-80-20 100.6 0.24 18.16
RE3NAR-80-20 106.5 0.29 17.92
RE5NAR-80-20 139.1 0.37 13.89
RE1NASH-110-3 105.7 0.26 17.75
RE3NASH-110-3 116.2 0.30 16.23
RE5NASH-110-3 149.6 0.40 10.25
a. Specific surface area, b. Pore volume, c. Average pore diameter

The X-ray diffraction patterns of the fresh Ni/Al HTlcs are shown in Figure 1. Sharp and symmetric
reflections for the basal (003), (006), (110) and (113), and less symmetric peaks in the (012), (015) and
(018) planes were the characteristic reflections of HTlcs. The results indicated that HTlcs with layered
structures were the main component in the samples, and no signals of other crystalline phases were
detected, which agreed with the literature reports [13,25,26]. Therefore, it can be concluded that HTlcs
with single crystal phase were successfully prepared by coprecipitation method used.
Molecules 2010, 15 5143

Figure 1. XRD patterns of Ni/Al HTlcs.
( 003)
5000
4000
y ( 006)
t
i 3000
s
n
e
t ( 012)
n
2000
I
( 015) ( 110)( 113)
( 018)
1000
0
10 20 30 40 50 60 70 80 90
2222θθθθ////((((°°°°))))

As shown in Figure 2, the characteristic reflections of HTlcs disappeared after reduction, but the
typical reflections of NiO and Ni were all detected by XRD. When Ni/Al HTlcs were reduced at
300 °C, the hydrogen promoted the decomposition of carbonate anions to CO andofNO - to NO, so
2 3
structural holes were generated on the surface of Ni/Al HTlcs, and these structural holes favored the
accessibility to the NiO particles [24]. The formation of a new crystal metallic Ni phase was attributed
to the reduction of NiO to Ni0 with the release of water after catalytic precursors were reduced at
300 °C [25]. The NiO and Ni features of the reduced HTlcs under stirring reflux were affected by the
Ni/Al molar ratio, the X-ray patterns of NiO became broad and asymmetric as the Ni/Al
ratio increased.
Figure 2. XRD patterns of Ni-based catalysts obtained from Ni/Al HTlcs: ●-NiO, ♦-Ni.
¤
)
.
u
••
.
a ¤
y ( • • ¤
it RE1NASH-110-3
s
n
RE3NASH-110-3
e
nt RE5NASH-110-3
I
RE1NAR-80-20
RE3NAR-80-20
RE5NAR-80-20
20 40 60 80
2222θθθθ////((((°°°°))))
Molecules 2010, 15 5144

Nevertheless, the Ni X-ray patterns became sharp and intense as the Ni/Al ratio decreased. On the
other hand, the low relative intensity of the XRD signals suggested a high metal dispersion after the
reduction treatment [33]. These changes were also observed obviously in the reduced Ni/Al HTlcs with
static hydrothermal treatment. In conclusion, the lamellar structure mostly collapsed with the
decomposition of CO 2- and NO - and the release of partial H O in the interlayer, which could be
3 3 2
proven by the disappearance of the characteristic reflections of HTlcs.
The FTIR analysis of different HTlcs and reduced HTlcs is shown in Figure 3. 1NAR-80-20,
1NASH-110-3 and 5NASH-110-3 displayed similar spectra showing typical hydrotalcite-like
compounds peaks and three general types of IR-active vibrations could be distinguished, which were
the molecular vibrations of hydroxyl groups, lattice vibrations of the octahedral layers and vibrations of
the interlayer anions [27]. The intense and broad band observed about 3,460 cm-1 was assigned to the
OH stretching of water molecules and hydroxyl groups of the brucitic layers. The absorption at 1,637
cm-1 was attributed to δ vibrationof interlayer water molecules. Moreover, the presence of the CO 2-
H2O 3
interlayer anions was manifested by the ν mode around 1,360 cm-1. Evidence for the presence of NO -
3 3
was also found with the characteristic vibrations of ν at 1,385 cm-1. Bands below 1,000 cm-1 were due
3
to stretching and deformation modes of M-O, M-O-M, and O-M-O moieties in the brucite-like layers
[28]. The peak at 620 cm-1 might be related to the presence of Ni cations and Ni–OH translation, the
peak at 565 cm-1 was due to the Al-OH translational mode, while the sharp and intense band at 430
cm-1 could be assigned to the condensed [AlO ]3- group or a single Al–O bond [29].
6
Figure 3. FTIR spectra of Ni/Al HTlcs and Ni-based catalysts.
) RE5NASH-110-3
.
u

a. RE1NASH-110-3
(

n RE1NAR-80-20
o
i 0 0
miss 15NNAASSHH-1-11100-3-3 142 108 0
6
s 4
n 1NAR-80-20
a
r
T 1637 20565430
3446 1384 6
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber(cm-1)

The spectra of the samples reduced at 300 °C showed that the bands at 3,460 and 1,637 cm-1 did not
obviously change. However, no bands corresponding to the interlayer nitrate anions were observed, the
band of CO 2- groups at 1,360 cm-1 also disappeared, which might be caused by the decomposition of
3
NO - anions to NO and partial conversion of CO 2- anions to CO , respectively. The results indicated
3 3 2
that the lamellar structure mostly collapsed, as also testified by the XRD patterns of the reduced Ni/Al
HTlcs. The absorption bands between 850 and 1,500 cm-1 were consistent with that of the calcinated
Molecules 2010, 15 5145

HTlcs, and two new bands at 1,420 and 1,080 cm-1 were found and assigned to the ν and ν modes of
2 3
the CO 2- anions [27]. Moreover, the bands of NiO were also observed at 460 cm-1 [30].
3
SEM morphology of the samples RE1NAR-80-20, RE1NASH-110-3, RE5NAR-80-20 and RE5N
ASH-110-3 is shown in Figure 4. The results showed that the lamellar structure of HTlcs was
destroyed and microporous structure of Ni-based catalysts was formed. When the Ni/Al molar ratio
increased from 1:1 to 5:1, the particle size of samples decreased conversely. The reduced sample of
Ni/Al molar ration 5:1 had a large specific surface area, which was also confirmed by specific surface
area analysis. Among the four samples, RE1NAR-80-20 exhibited homogeneous spherical structure,
the morphology RE1NASH-110-3 of was granular or triangular, whereas RE5NAR-80-20 appeared
agglomerated and RE5NASH-110-3 presented a honeycombed morphological structure. From these
results, it could be concluded that Ni-based catalysts had randomly interconnected micropores, which
was in accordance with the results reported in the literature [23]. The morphological changes of the
resultant samples might be due to different Ni/Al molar ratio and variation of crystallization
conditions.
Figure 4. SEM photographs of Ni-based catalysts: (a) RE1NAR-80-20; (b) RE1NASH-
110-3; (c) RE5NAR-80-20; (d) RE5NASH-110-3.

(a) (b)

(c) (d)
Molecules 2010, 15 5146

2.2. Catalytic hydrogenation of ethyl acetate to ethanol
The activity and selectivity of catalytic hydrogenation of ethyl acetate to ethanol on Ni-based
catalysts are shown in Table 3. According to the hydrogenation reaction stoichiometry, one molecule of
ethyl acetate would be reduced to two molecules of ethanol. However, hydrogenation reaction by-
products such as butyl acetate, acetaldehyde, ethyl ether, methane and ethane were also detected.
Among these by-products, butyl acetate might be produced by ester-exchange reaction between the
original ester and the alcohol used as solvent, a phenomenon also reported in the literature [10]. In all
Ni-based catalysts from Ni/Al hydrotalcite-like compounds, the conversion of ethyl acetate was similar
and around 90%, but the different catalysts showed different selectivity for ethanol. Nickel as a base
metal catalyst could offer much promise for the hydrogenation of organic compounds including ester
reduction [31]. Interestingly catalysts with high proportions of Ni didn’t stand for their high
hydrogenation activity. Monzon et al. reported that the conversion rate, selectivity and yield for the
hydrogenation of acetylene to ethylene decreased as the Ni concentration increased [28]. This
phenomenon was also seen in our experiments.
Table 3. Hydrogenation of ethyl acetate on Ni-based catalysts.
Catalyst Conversion Selectivity
Ethanol Butyl acetate Ethyl ether Acetaldehyde
RE1NAR-80-20 90.6 64.6 19.3 0.4 6.2
RE3NAR-80-20 91.6 61.5 21.5 0.3 8.6
RE5NAR-80-20 90.4 59.1. 23.7 0.3 8.9
RE1NASH-110-3 90.5 68.2 14.5 0.7 4.3
RE3NASH-110-3 90.3 64.3 17.3 0.5 6.8
RE5NASH-110-3 89.2 62.5 18.9 0.4 8.5
Nickel powder 76.3 70.3 14.5 0.6 3.8
Reaction conditions: catalyst dosage 0.14 g, temperature 250 °C, time 9 h, hydrogen pressure
6 MPa (at 25 °C), ethyl acetate 0.02 mol.

Results from the experiments also indicated that the selectivity for ethanol fell with the rise in Ni/Al
mole ratio, whereas the selectivity for butyl acetate and acetaldehyde rose conversely. The Ni-based
catalyst RE1NASH-110-3 showed the highest selectivity and yield of ethyl acetate to ethanol, which
were 68.2% and 61.7%, respectively. The results were lower than the results of 99% selectivity and
79 wt% yield of ethanol reported by Bournonville et al. with Raney nickel and tetrabutyltin, but were
higher than the results of 49.5% conversion and 46.5 wt% yield of ethanol with 2.5%Ni/3%Sn/SiO
2
[32]. The conversion of ethyl acetate and yield of ethanol with pure nickel powder were 76.3% and
53.6%, respectively, ranking as the lowest yield for ethanol among the Ni-based catalysts examined in
these experiments. In conclusion, RE1NASH-110-3 is an efficient catalyst for the catalytic
hydrogenation of ethyl acetate to ethanol.
Molecules 2010, 15 5147

2.3. Reaction pathway for the hydrogenation of ethyl acetate to ethanol
According to the experimental results and the related literatures, a plausible reaction pathway for
hydrogenation of ethyl acetate to ethanol is proposed as Scheme 2. In the pathway, ethyl acetate is
firstly absorbed on the active Ni2+ cation sites in association with aluminium oxide through a relatively
weak acyl structure [33], the metal active sites in the catalysts are responsible for the carbonyl
activation, which also seems to be the reason for the increased conversion. The weak acyl structure
facilitates desorption of the resultant products from the surface of the catalysts. Alumina without active
centers is a better support for dispersing nickel crystallites (Ni0). Ni0 as another active sites is very
active for adsorbing dissociative H and may supply Ni2+ cations with activated atomic hydrogen to
2
obtain nickel hydride, and the transfer of the hydrogen atom in nickel hydride to the carbonyl carbon
atom forms hemiacetal groups according to the Adkins model [34]. Subsequently, acetaldehyde and an
adsorbed ethoxy group are obtained through the cleavage of the C-O bond of the hemiacetal group, and
then the ethoxy group would be quickly hydrogenated to ethanol. In addition, side reactions of ethanol
also happen, for example, reversible dehydrogenation of ethanol to aldehydes, hydrogenation and
dehydration of ethanol to methane and ethane, inter-molecular dehydration of ethanol to ethyl ether,
ester-exchange reaction between the original ester and the solvent alcohol to butyl acetate. Hence,
hydrogenation of ethyl acetate over Ni-based catalysts not only leads to the formation of ethanol but
also the formation of a variety of by-products such as acetaldehyde, methane, ethane, ethyl ether and
butyl acetate. How to control the adverse side-reactions and increase the yield of ethanol is the subject
of further studies.
Scheme 2. The proposed reaction pathway for hydrogenation of ethyl acetate.
H3C O H C O
3
C2H5 C2H5
O C +H O C
H H 2 H
H
Ni0/Al O Ni O Ni0/Al O Ni O
O Ni- +b aHs e2d c ataly st C2H5 O
H O H H3C H
H3C O CH3 Ni0/Al O Ni O
+
C H2
H
3(CH2)3OH +H2 _ +H2
O
H3C OH
H3C O CH3 H2 -
+ H
2O
O
H2
-
CH4 + H3CCH3 H3C O CH3
Molecules 2010, 15 5148

3. Experimental
3.1. Materials
Nickel powder, ethanol and butyl acetate (all >99% purity) were purchased from Aladdin
Chemicals. Acetaldehyde and ethyl ether purchased from BASF Chemical Company were analytical
grade and filtered before use. Other reagents and chemicals were all of analytical grade, obtained from
Sinopharm Chemical Reagent Co. Ltd and used directly without any further purification.
3.2. Preparation of Ni/Al HTlcs
Six samples of Ni/Al hydrotalcite-like compounds under two different crystallization conditions
were prepared by a coprecipitation reaction at constant pH value. A mixture of solutions of Ni (NO ) ·
3 2
6H O and Al (NO ) ·9H O with Ni/Al molar ratios of 1, 3 and 5, respectively, was added along with
2 3 3 2
1M of NaOH and Na CO solutions to a beaker containing deionized water (approximately 100 mL) at
2 3
a constant pH of 8.5 ± 0.2. The solutions were added dropwise under vigorous stirring at room
temperature over about 4 h. The resultant suspension was stirred continuously under reflux for 20 h at
80 °C or maintained under static conditions (static hydrothermal treatment) for 3 h at 110 °C in a
stainless steel autoclave, respectively. The products were cooled to room temperature, filtered and
washed thoroughly with deionized water, then dried for 16 h at 100 °C. The samples were reduced in a
stainless steel autoclave at 300 °C for 4 hours, at 4 MPa of hydrogen pressure and with a stirring speed
of 600 rpm. In the end, the Ni2+ in the samples was thus reduced to Ni0, as confirmed by the color
change from the initial green color to gray after the reaction.
The resultant samples were named as follows: MNAR-T-t and MNASH-T-t for the hydrotalcite-like
compounds samples under stirring reflux and static hydrothermal treatment, respectively. M of 1, 3 and
5 stand for the series with Ni/Al molar ratio of 1/1, 3/1 and 5/1, respectively. T is the heating
temperature (80 or 110 °C), and t is the crystallization time (3 or 20 h). REMNAR-T-t and
REMNASH-T-t are the reduced HTlcs.
3.3. Characterization of catalysts
The metal contents of the catalysts were determined with an inductively coupled plasma-atomic
emission spectrometer (IRIS 1000, Thermo Jarrell Ash) under standard conditions. Ni/Al hydrotalcite-
like compounds were digested with a mixture of concentrated HNO and HClO , and diluted with 2%
3 4
of HNO .
3
X-ray diffraction (XRD) analysis was performed on a Rigaku powder diffractometer (Rigaku,
Japan) with Cu K radiation at 40 kV and 40 mA. Scans were performed over the 2θ ranges from 3° to
α
90° at a step of 0.02° and a count time of 17.7 s at each step. The crystalline phases were identified by
Powder Diffraction Standards (JCPDS) files.
Fourier transform infrared (FTIR) spectroscopy of Ni/Al HTlcs and their reduced products were
recorded over the range from 400 to 4,000 cm-1 on a SpectrumGX FT-IR spectrometer. The powder
samples were mixed with KBr and pressed into thin wafers before being measured.